Problem: Which of these numbers is composite? ${29,\ 51,\ 53,\ 59,\ 61}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 29, 53, 59, and 61 each have only two factors. The factors of 51 are 1, 3, 17, and 51. Thus, 51 is the composite number.